An acoustic wave device can be used as a filter element or oscillator in, for example, a television receiver, a mobile phone terminal, or a PHS (Personal Handy-phone System). An FBAR (Film Bulk Acoustic-wave Resonator) and a SAW (Surface Acoustic Wave) device are widely-known examples of devices that make use of acoustic waves (e.g., see JP 2004-112748A, International Publication WO 98/52279, and JP 2008-113061A). ASAW device can be used in various types of circuits in an apparatus that processes wireless signals in, for example, the 45 MHz to 2 GHz frequency band. A SAW device can be used in, for example, a transmission bandpass filter, a reception bandpass filter, a local oscillation filter, an antenna sharing device, an IF (Intermediate Frequency) filter, or an FM (Frequency Modulation) modulator.
In recent years, there has been an increase in the performance of mobile phone terminals and the like, and along with this, in the exemplary case of a SAW device used in a bandpass filter, there has been demand for improvements in characteristics such as low-loss in the passband, high suppression outside the passband, and temperature stability, as well as demand for a smaller device size. Among these characteristics, improving temperature stability has become an urgent issue in light of the increase in the power density of devices that is due to a reduction in device size, an increase in input power to devices, and the like. To address the temperature stability issue, there have been proposals for improvements by devices whose structures are different from conventional SAW devices of recent years. Examples of a device in which temperature stability is improved include a boundary wave device and a Love wave device in which interdigital electrodes are formed on a piezoelectric substrate, and furthermore a thick dielectric element is formed so as to cover the interdigital electrodes (e.g., see JP 2008-79227A).
The electromechanical coupling coefficient k2 is an important parameter that determines the characteristics of an acoustic wave device. Although the electromechanical coupling coefficient k2 is a constant that is determined by the substrate of the acoustic wave device, in actuality the electromechanical coupling coefficient k2 changes slightly according to, for example, the width dimension and thickness dimension of the interdigital electrodes. Also, when an acoustic wave device filter is manufactured, the frequency characteristics of the filter change slightly according to, for example, the width dimension and thickness dimension of the interdigital electrodes. In order to adjust the electromechanical coupling coefficient k2 and frequency characteristics of the acoustic wave device, it is sufficient to adjust, for example, the width dimension and thickness dimension of the interdigital electrodes. However, since the interdigital electrodes in Love wave devices and boundary wave devices are covered by a thick dielectric element, it is difficult to adjust the width dimension and thickness dimension of the interdigital electrodes in order to adjust the electromechanical coupling coefficient k2, the frequency characteristics, and the like after device manufacturing.